1. Field of the Invention
The present invention relates to a method for forming an oxide film in semiconductor devices, and more specifically, to a method for forming a gate oxide film of a transistor or a tunnel oxide film of a flash device, which requires a high quality.
2. Discussion of Related Art
A tunnel oxide film of a conventional flash EEPROM device and a gate oxide film of other memory devices or a logic device require the quality of a high level in view of their applications. Main factors that decide the oxide film of a high quality include the density of interface trap charge and oxide trap charge.
Interface trap charge collectively exists at the boundary where a silicon (Si) substrate and an oxide film are adjacent to each other. Oxide trap charge exist both at the bulk of the oxide film and the boundary of the Si substrate and the oxide film. However, charge in which electrons are trapped mainly exists at the bulk and charge in which holes are trapped typically exists at the boundary of the Si substrate and the oxide film.
Therefore, in order to form the oxide film of a high quality, it is an important technical problem to reduce the density of the interface trap charge and the oxide trap charge. In other words, if an oxide film having a high density of interface trap charge and oxide trap charge is used as a gate oxide film and a tunnel oxide film, there is a problem in that the threshold voltage drift of a transistor occurs due to electric charge trapped in the trap charge. Furthermore, there occurs a problem that breakdown of the oxide film occurs instantly.